Frequency modulation system



Aug. 24, 1943. w. H, WIRKLER FREQUENCY MODULATION SYSTEM Filed Dec.

m@ wkn' WAZTE'R MUR/(FR- Patented Aug. 24, 1943 UNITED STATES PATENT OFFICE FREQUENCY MoDULA'rroN SYSTEM- Walter H.. Wirkler, Cedar Rapids, Iowa, assignor to Collins Radio Company, Cedar Rapids, Iowa, a corporation o! Iowa Application December 13, 1940, Serial'No. 370,063

A `8 Claims.

My invention relates broadly to frequency modulation systems for radio frequency transmitters, and more particularly to a system of modulation applied to polyphase components of fixed frequency, resulting in frequency modulation of frequency moduation is required, this oscillatorY required to obtain a measure of this frequency driftl and to actuate compensating frequency controlling means. This arrangement is open to the objection that failure of this auxiliary equip- Vment will allow the main oscillator to drift far from its assigned frequency.

In the Armstrong system, the main oscillator is of constant frequency and under direct crystal control. Armstrong, however, does not obtain direct frequency modulation in the following circuits. Instead, he obtains phase modulation which is made the equivalent of frequency modulation by means of simple frequency weighting networks in the audio circuits. The objection to the Armstrong system is that the degree of phase modulation obtainable is rather limited, so that a large number of frequency multiplying stages must follow the modulating stage to obtain the required degree of modulation. i

One of the objects of my invention is to provide a system of frequency modulation veffected by variation of the phase of a carrier of unlimited phase rotation derived from polyphase components of fixed frequency, so that the system of my invention may be termed a phase spinner type of frequency modulation.

Another object of my invention is to provide control means for the polyphase components of the phase spinner type of frequency modulating system, for maintaining the composite frequency modulated wave constant in amplitude.

A further object of my invention is to provide Y frequency modulation of a radio transmitter in whichthe mean carrier frequency is supplied directly by .a constant frequency oscillator which may be under direct crystal control, and the instantaneous deviation from this mean frequency is proportional to and directly inuenced by an audio modulating voltage.

' Still another object of my invention is to provide radio frequency excitation to the modulating stage from a constant frequency crystal controlled oscillator, as in the Armstrong system, but to obtain directly a relatively high degree of frequency modulation instead of a relatively small degree of phase modulation, thereby eliminating the need of a large degree of frequency multiplication following the modulator.

Other and further objects of my invention reside in the methodof operation and the arrangement in the system hereinafter described with reference to the accompanying drawing, which is a schematic diagram of a radio transmitteraemploying one form of the frequency modulation system of my invention. A constant frequency crystal controlled oscillator is indicated at A, operating directly on the assigned frequency of the transmitter or, preferably, on a submultiple thereof with the use of a limited degree of frequency multiplication following modulation. The oscillator A excites a balanced modulator` D directly and excites a balanced modulator C through a ninety-degree phase shifting `network B. The outputs of balanced modulators C and D are combined in a bridge circuit E which prevents interaction between t'he circuits at C and D.

The output of bridge circuit E is supplied to the output portion of the transmitter, which may contain certain frequency multiplying stages at F. The output of bridge circuit E is also supplied to a balanced modulator J where it is amplitude modulated in balanced relation by audio voltages from terminalsI.

The output of modulator J goes through phase adjusting network K to rectifier circuits G and H Where it is combined withexcitation voltages from the grid circuits of modulators C and D respectively. The outputs of rectiers G and H supply modulating voltageto balanced modulators C and D respectively.

The operation of the system depends upon the fact that two balanced modulators whose outputs are combined and whose inputs are excited with carrier frequency voltages ninety degrees out of phase can produce a complete phase rotation of the carrier frequency in their combined output when their grid circuits are supplied with controlling voltages ci the proper character in addition to radio frequency excitation voltage. lt is the function of elements J, l, G and H to generate these controlling voltages.

In addition to the output from modulator J, rectiers G and H are supplied through phase adjusting network K with the output of a modulator M which is excited in phase quadrature with modulator J. In the absence of audio input voltage at terminals I, balanced modulator J passes no radio frequency current through to its output circuit. However, modulator M is under the control of a biased rectifier L. The phase adjusting network K is adjusted so that the radio frequency output of modulator M is in such phase that when applied to rectilers G and H it causes the output from modulators C and D to grow in amplitude. This will cause the carrier amplitude to grow until it exceeds the bias on rectifier L, after which the output of modulator M will be reduced to that required to make up the leakage from condensers i and 2 and/or 3 and li, in the circuits of rectiers G and H, thro-ugh leak resistors and 6 and/or and 8. Balanced modulators C and D then transmit through output circuit E a constant frequency carrier voltage of exactly the same frequency as the oscillator A which drives them.

During the instant in an audio cycle when a voltage V is applied diiferentially across the grids of balanced modulator J, some carrier frequency voltage of a certain phase and of amplitude instantaneously proportional to voltage V is passed. Phase shift network K is so adjusted that when this voltage is combined with the two-phase carrier frequency energy in rectiers G and H, either or both of said rectiflers begin to charge condensers I and 2 and/or 3 and Il respectively in differential relation, depending on the radio frequency phase of the voltage from the network K at this instant. The network K can be adjusted so that the rate and direction of charging of condensers I, 2, 3 and i is such that the net increment in the combined outputs of modulators C and D is a radio frequency voltage ninety degrees displaced from the voltage that is already there. In other words, the end of the radio frequency vector in the output circuit is being moved in a circle. Since this motion is continuous as long as the voltage V exists, and the rate of change of phase is proportional to V, the vector may turn over QV times per second, where Q is a constant. This amounts to a frequency shift of f cycles per second, where f=QV.

Furthermore, if the direction of voltage V is reversed, the phase of the radio frequency output of modulator J is reversed and, consequently, the incremental voltage in the combined outputs of modulators C and D is reversed, making the vector rotate the other way and resulting in a frequency shift in the opposite direction. Hence, frequency modulation of the mean carrier frequency supplied by oscillator A is achieved. This adjustment of phase shift network K is the same as that required for modulator M to change the amplitude only, since modulators J and M are supplied with input radio frequency voltage in phase quadrature.

Referring to the drawing in more detail, the elements shown in block form, such as the crystal-controlled oscillator at A, the phase shifting or adjusting networks at B and K, and the frequency multipliers and amplifiers at F are al1 of conventional arrangement, and any suitable devices of such description may be employed. The

CII

output ci phase shifting means B is fed to an output coil IG intercoupled with coil Il in the input of the balanced modulator C and coil I2 in the circuits of the balanced rectifier G. On the other hand, an output coil i4 is connected directly to oscillator A and intercoupled with coil I5 in the input of balanced modulator D and coil I6 in the circuits of the balanced rectier H. Carrier energy is thus supplied to balanced modulators C and D, for modulation therein and transmission through -the respective output circuits at I? and i8 to the bridge circuit E; thus also, carrler energy is supplied to the balanced rectifier circuits G and H for demo-dulating the output of modulator J and the phase adjusting network K in proper phase for then modulating the carrier energy in balanced modulators C and D. The output of modulator J and the phase adjusting network K is applied to the balanced rectiers G and H through center tap connections at the coils I2 and I6, respectively, through blocking condensers I9 and 20.

The rectifier tubes are shown at G1 and G2 in rectifier G and at H1 and H2 in rectier H, and are effective upon operation respectively to charge condensers I, 2, 3 and 4. With carrier energy supplied through coils I0, Ii and I2 in phase with modulated energy applied at the center tap on coil I2, tubes G1 and G2, for example, constitute a balanced rectifier which can act simultaneously to charge one and discharge the other of condensers I, 2, depending on the sense of the modulated radio frequency voltage with respect to the carrier. The voltage across condensers I and 2, resulting from this accumulated charge, then serves as modulating potential applied through leads 2l and 22 to the balanced modulator C in proper relation to the carrier energy supplied through coil I0 to the input coil II of the modulator C. Rectifier tubes H1 and H2 are similarly operative under different phase relations of the carrier energy from oscillator A and the modulated energy applied through the blocking condenser 2D.

The output circuits of balanced modulators C and D include the resonant circuits l1 and I8 connected to a common source of power at 23 from which screen grid potentials also are derived through lead 24. The bridge circuit E includes coil 25 coupled with resonant circuit 'l and having a center tap connection to coil 26 which is coupled with resonant circuit I8. The difference of potential across coil 26 is thus applied to parallel paths through coil 25, one being a resistance connection 21 to ground and the other a connection through coupling coil 28 to ground. The frequency modulated output of the bridge circuit E is delivered to frequency multiplying means and ampliiiers at F, substantially in parallel with coil 28, and the nnal output is delivered through coupling means ai; 3|. Coil 28 supplies high frequency energy to the modulator J through coil 29 connected in the input circuit of the modulator, and also to modulator M through coil 30 connected in the respective input circuit, with the energy derived through coil 30 in phase quadrature with respect to that obtained through coil 29. i

The balanced modulator J, as shown, comprises two triodes J1 and J2 having the grids thereof energized in opposite phase relation and the anodes energized by modulation energy through transformer 32, also in opposite phase relation, from the audio input I. The output of the modulator J is coupled through resonant means 33 to coil 34 for application to the phase adjusting net- Work K.

` The modulator M embodies an electron tube having its control grid supplied with high frequency energy from coil 30 through resonant circuit 35, while at the same time control bias potential is applied through lead 36 from the limiting rectifier L which employs a diode connected with the output coupling means at 3I and biased by source 31 to rectify energy only above a certain level. The output of rectier L above such level is applied `as a cut-off bias to the control grid of modulator M; but until such level is reached, energy is transmitted through modulator M and its tuned'outputcircuit at 38 to coupling coil 39 which is in circuit with coil 34 in the input of the phase adjusting network K.

The operation of the modulation system of my invention may be more particularly described as follows'. Forgetting for the moment losses and misadjustments which necessitate the use of the limiter L and modulator M, the phase adjustments are made such that the output from modulator C at one instant in the phase turn-over cycle can come back through elements E, J, and K, so that it appears as a crystal frequency voltage at the center tap of the coil I2 in rectifier G, ninety degrees displaced in phase-leading or lagging, depending on the direction in which modulator J is unbalancedfrom the voltage ini duced in this coil I2 and the coil II in modulator C from coil I 0. No voltage such as to charge condenser I and discharge condenser 2 (or the reverse) is thus generated, and the feedback loop therefore does not cause modulator C to react on itself. The feedback voltage applied to the center tap of the coil I6 in rectifier H, however, lis in phase withk that induced in coil I 6 by the coil I in modulator D, and the rectifiers in H tend to charge condenser 3 and discharge condenser 4 or the reverse, depending again on the relative sense of the two voltages-the rate of charge and discharge depending on the magnitude of the voltage 'being fed back from modulator J. Carrying this on, it is seen that the output from each balanced modulator does not affect the rate of change of its own grid voltage but does affect that of the other balanced modulator.

Now, let

Ec=the difference in voltage between the grids of balanced modulator C, taking an arbitrary direction as positive; and Ed=the difference in voltage between the i grids of balanced modulator D.

which means that the rate of change of each voltage is proportional to the other voltage, Where the proportionality factor A may be either positive or negative and its amplitude and sig-n depend on the direction and degree of unbalance in the modulator J. l The unique solution of this set of differential equations is Ed=cos At forgetting arbitrary constants. This shows conclusively that the two alternating voltages applied dierentially to the grids of balanced modulators C and D are sinusoidal, in phase quadrature, and of frequency directly proportional to the amplitude of the voltage fed back from modulator J.

After mixing the outputs of two balanced modulators supplied with radio frequency carrier voltages of frequency X in phase quadrature and modulated with alternating voltages of frequency also in phase quadrature, the combined output of the two modulators consists of a single frequency, say

Which is displaced from the crystal frequency by an amount proportional to A, which may be either` positive or negative in this case, thereby affecting the phase order of the polyphase modulating voltages and the direction of the frequency displacement.

In the discussion above it was noted that the output of neither modulator C nor modulator D affected its own input. This is true with perfect adjustment. If the adjustment isl not perfect, the output may affect the rate of change of the input, and degeneration or regeneration will result in excessive and undesired amplitude modulation. The radio frequency phasing of modulator M has purposefully been made such that when modulator M is unbalanced, the outputs of modulators C and D will affect their own inputs giving regeneration or degeneration, depending on the direction in which modulator M is unbalanced. Modulator M is controlled by the limiting rectifier L in such lmanner that it tends to oppose regenerative or degenerative tendencies and thus to hold the amplitude modulation down and establish the carrier amplitude at a fixed reference level.

The system has inherent mean frequency stability. For an actual transmitter design, it is suggested that the crystal controlled oscillator be approximately 4000 kc. One tripler and two doublers following the modulators would give an output frequency of 48 megacycles. If a modulation of :1 -100 kc. were wanted at 48 mc. the modulator would need to furnish i8 kc.

The overall number of tubes and circuits is much below that required by Armstrongs system. Further, the frequency stability is much above that of the free oscillator. Frequency compensation can be applied to this system in a manner similar to that employed with the free oscillator type of frequency modulation now in use. A suitable discriminator may have its output connected to unbalance modulator J in such a direction as to compensate for any carrier frequency drift.

While I have described my invention in a preferred embodiment, I desire it understood that no limitations upon my invention are intended thereby and that modifications may be made inv various respects within the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as follows:

l. A frequency modulation system comprising means for generating polyphase currents of fixed frequency, a balanced modulator individual to each phase of said polyphase currents and energized by energy of the respective phase derived roxnsaid generating means, a bridge circuit fdr combining the outputs of the respective modulators, to produce a derived output current of said fixed frequency, means providing unlimited control of the phase of said derived output current comprsing independent modulation circuits for the bal nced modulators of each respective phase, phase rotating means actuated by said derived output current and by said polyphase currents of fixed frequency for rotating the phase of said derived output current by sup- Flying modulating energy to said modulation circuits, and means responsive to a source of signalling current and connected to said modulation control means wherein the rate of phase rotation is controlled by said signalling current, the phase rotation constituting an effective irequency deviation from said xed frequency.

2. A frequency modulation system as set forth in claim l wherein said phase rotating means includes an auxiliary balanced modulator excited by said derived output current and audio frequency modulation energy, a phase shifting network for adjusting the phase of the modulated energy from said auxiliary balanced modulator, and detector means individual to each said balanced modulator and cooperatively energized by fixed frequency excitation from the respective phases of said polyphase currents and by the phase adjusted modulated energy from said auxiliary modulator to provide the modulation energy required for the 'respective balanced modulators, the phase of said phase adjusted modulated energy being such that the modulation energy supplied to any balanced modulator is derived through said auxiliary modulator and detector means from a component supplied to the bridge circuit by a balanced modulator other than the one supplied with modulation energy.

3. A frequency modulation system as set forth in claim 1 and including level control means comprising an electron tube circuit excited by energy from said'bridge circuit and controlled by the level of frequency modulated energy in the output of said frequency multiplying means, phase shifting means for adjusting the phase of the energy from said electron tube circuit, and rectifier means individual to each said balanced modulator and cooperatively energized by xed frequency excitation from the respective phases of said polyphase currents and by the phase adjusted output of said electron tube circuit to provide direct control -voltages eifective on the respective modulators to control the level of the modulated'output energyl the phase of said phase adjusted output of said electron tube circuit being such that the control voltage eifective on any balanced modulator is derived through said electron tube circuit and rectifier means from a component supplied to said bridge circuit by the same balanced modulator.

4. In a frequency modulation system including a source of carrier frequency energy, means for producing a derived current of said carrier frequency subject to complete phase rotation, said means comprising a pair of balanced modulator circuits each having high and low frequency input circuits and a tuned output circuit, means for supplying carrier frequency energy in phase quadrature to said high frequency input circuits, and a combining circuit wherein said derived current is produced by the combination of output currents from both balanced modulators; phase rotating means for rotating the phase of said derived current comprising means actuated by said derived current and by carrier current for supplying modulation energy to said low frequency input circuits in phase quadrature; and means responsive to a signalling current and connected to said phase rotating means wherein the rate of phase rotation is controlled by said signalling current, said phase rotation of said derived current constituting an eii'ective frequency deviation from said carrier frequency. A

5. A frequency modulating system as set forth in claim 4 wherein said means for supplying modulation' energy in phase quadrature to the respective modulator circuits includes an auxiliary balanced modulator excited by energy from said combining circuit and by primary modulation energy, phase shifting means for adjusting the phase of the modulated energy from said auxiliary modulator, and detector means individual to each of said balanced modulator circuits and cooperatively energized by carrier energy of the respective phases and by the phase adjusted modulated energy from said auxiliary modulator to provide the modulation energy required ior the respective balanced modulator circuits, the phase of said phase adjusted modulated energy being such that the modulation energy supplied 'to one balanced modulator circuit is derived through said auxiliary modulator and said detector means from a carrier component supplied to the combining circuit from the other of said .pair of balanced'modulator circuits.

6. A frequency modulating system as set forth in claim 4 and including carrier level control means comprising an electron tube circuit excited by energy from said combining circuit and controlled by the level of frequency modulated energy in said output circuit means, phase shifting means for adjusting the phase of the energy from said electron tube circuit, and rectifier means individual to each said balanced modulator circuits and cooperatively energized by carrier energy of the respective phases and by the phase adjusted output of said electron tube circuit to provide direct control voltages effective in the respective balanced modulator circuits to control the level of the amplitude modulated sideband energy delivered to said combining circuit, the phase of said phase adjusted output of said electron tube circuit being such that the control voltage effective on one of said balanced modulator circuits is derived through said electron tube circuit and said rectifier means from a carrier component supplied to said combining circuit by the same balanced modulator circuit.

7. A frequency modulating system as set forth in claim 4 and including carrier level control means comprising an electron tube circuit excited by energy from said combining circuit and controlled by the level of frequency modulated energy in said output circuit means, phase shifting means for adjusting the phase of the energy from said electron tube circuit, and rectier means individual to each said balanced modulator circuits and cooperatively energized by car rier energy of the respective phase and by the phase adjusted output of said electron tube circuit to provide direct control voltages effective in the respective balanced modulator circuits to control the level of the amplitude modulated sideband energy delivered to said combining circuit, the phase of said phase adjusted output of said electron tube circuit being such that the control voltage effective on one of said balanced modulator circuits is derived through said electron tube circuit and said rectifier means from a carrier component supplied to said combining circuit by the same balanced modulator circuit; the said means for supplying modulation energy in phase quadrature to the respective modulator circuits, in said frequency modulating system, including an auxiliary balanced modulator excited by energy from said combining circuit in phase quadrature with the excitation of said electron tube circuit and by primary modulation energy, the modulated output of said auxiliary modulator being supplied to said phase shifting means in common with the energy from said electron tube circuit ,and being detected in said rectifier means to provide the modulation energy required for the respective balanced modulator circuits. the phase adjustment effected in said phase shifting means being such that the modulation energy supplied to one balanced modulator circuit is derived through said auxiliary modulator and said rectifier means from a carrier component supplied to the combining circuit from the other of said pair of balanced modulator circuits.

8. In an electrical system for generating two phase alternating current the frequency of which is proportional t0 the instantaneous amplitude of a controlling current and the phase order of which is determined by the polarity of the controlling current, means comprising a pair of amplifying circuits each having input connections and output connections, means for supply-l ing the input of the first said amplifying circuit with a voltage proportional to the integrated value of output voltage in the second said amplifying circuit, means for supplying the input of said second amplifying circuits with voltage proportional to the integrated value of output Voltage in said rst amplifying circuit, and gain control means actuated by said controlling current and operable on both of said amplifying circuits alike.

WALTER H. WIRKLER. 

